Problem: A circle with circumference $18\pi$ has an arc with a $180^\circ$ central angle. What is the length of the arc? ${18\pi}$ ${180^\circ}$ $\color{#DF0030}{9\pi}$
Answer: The ratio between the arc's central angle $\theta$ and $360^\circ$ is equal to the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{360^\circ} = \dfrac{s}{c}$ $\dfrac{180^\circ}{360^\circ} = \dfrac{s}{18\pi}$ $\dfrac{1}{2} = \dfrac{s}{18\pi}$ $\dfrac{1}{2} \times 18\pi = s$ $9\pi = s$